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  • 1. Akrami, Yashar
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Solomon, Adam R.
    The nature of spacetime in bigravity: Two metrics or none?2015In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 47, no 1, p. 1838-Article in journal (Refereed)
    Abstract [en]

    The possibility of matter coupling to two metrics at once is considered. This appears natural in the most general ghost-free, bimetric theory of gravity, where it unlocks an additional symmetry with respect to the exchange of the metrics. This double coupling, however, raises the problem of identifying the observables of the theory. It is shown that if the two metrics couple minimally to matter, then there is no physical metric to which all matter would universally couple, and that moreover such an effective metric generically does not exist even for an individual matter species. By studying point particle dynamics, a resolution is suggested in the context of Finsler geometry.

  • 2. Amendola, Luca
    et al.
    Appleby, Stephen
    Avgoustidis, Anastasios
    Bacon, David
    Baker, Tessa
    Baldi, Marco
    Bartolo, Nicola
    Blanchard, Alain
    Bonvin, Camille
    Borgani, Stefano
    Branchini, Enzo
    Burrage, Clare
    Camera, Stefano
    Carbone, Carmelita
    Casarini, Luciano
    Cropper, Mark
    de Rham, Claudia
    Dietrich, Joerg P.
    Di Porto, Cinzia
    Durrer, Ruth
    Ealet, Anne
    Ferreira, Pedro G.
    Finelli, Fabio
    Garcia-Bellido, Juan
    Giannantonio, Tommaso
    Guzzo, Luigi
    Heavens, Alan
    Heisenberg, Lavinia
    Heymans, Catherine
    Hoekstra, Henk
    Hollenstein, Lukas
    Holmes, Rory
    Hwang, Zhiqi
    Jahnke, Knud
    Kitching, Thomas D.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kunz, Martin
    La Vacca, Giuseppe
    Linder, Eric
    March, Marisa
    Marra, Valerio
    Martins, Carlos
    Majerotto, Elisabetta
    Markovic, Dida
    Marsh, David
    Marulli, Federico
    Massey, Richard
    Mellier, Yannick
    Montanari, Francesco
    Mota, David F.
    Nunes, Nelson J.
    Percival, Will
    Pettorino, Valeria
    Porciani, Cristiano
    Quercellini, Claudia
    Read, Justin
    Rinaldi, Massimiliano
    Sapone, Domenico
    Sawicki, Ignacy
    Scaramella, Roberto
    Skordis, Constantinos
    Simpson, Fergus
    Taylor, Andy
    Thomas, Shaun
    Trotta, Roberto
    Verde, Licia
    Vernizzi, Filippo
    Vollmer, Adrian
    Wang, Yun
    Weller, Jochen
    Zlosnik, Tom
    Cosmology and fundamental physics with the Euclid satellite2018In: Living Reviews in Relativity, E-ISSN 1433-8351, Vol. 21, p. 1-345, article id 2Article, review/survey (Refereed)
    Abstract [en]

    Euclid is a European Space Agency medium-class mission selected for launch in 2020 within the cosmic vision 2015-2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and red-shifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

  • 3. Ashoorioon, Amjad
    et al.
    Casadio, Roberto
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Anisotropic non-gaussianity from rotational symmetry breaking excited initial states2016In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 12, article id 002Article in journal (Refereed)
    Abstract [en]

    If the initial quantum state of the primordial perturbations broke rotational invariance, that would be seen as a statistical anisotropy in the angular correlations of the cosmic microwave background radiation (CMBR) temperature fluctuations. This can be described by a general parameterisation of the initial conditions that takes into account the possible direction-dependence of both the amplitude and the phase of particle creation during inflation. The leading effect in the CMBR two-point function is typically a quadrupole modulation, whose coefficient is analytically constrained here to be vertical bar B vertical bar less than or similar to 0.06. The CMBR three-point function then acquires enhanced non-gaussianity, especially for the local con figurations. In the large occupation number limit, a distinctive prediction is a modulation of the non-gaussianity around a mean value depending on the angle that short and long wavelength modes make with the preferred direction. The maximal variations with respect to the mean value occur for the con figurations which are coplanar with the preferred direction and the amplitude of the non-gaussianity increases (decreases) for the short wavelength modes aligned with (perpendicular to) the preferred direction. For a high scale model of inflation with maximally pumped up isotropic occupation and epsilon similar or equal to 0. 01 the difference between these two configurations is about 0. 27, which could be detectable in the future. For purely anisotropic particle creation, the non-Gaussianity can be larger and its anisotropic feature very sharp. The non-gaussianity can then reach f(NL)similar to 30 in the preferred direction while disappearing from the correlations in the orthogonal plane.

  • 4. Ashoorioon, Amjad
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hemispherical anomaly from asymmetric initial states2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 4, article id 043009Article in journal (Refereed)
    Abstract [en]

    We investigate if the hemispherical asymmetry in the CMB is produced from asymmetric excited initial conditions. We show that in the limit where the deviations from the Bunch-Davies vacuum are large and the scale of new physics is maximally separated from the inflationary Hubble parameter, the primordial power spectrum is modulated only by position-dependent dipole and quadrupole terms. Requiring the dipole contribution in the power spectrum to account for the observed power asymmetry, A = 0.07 +/- 0.022, we show that the amount of quadrupole terms is roughly equal to A(2). The mean local bispectrum, which gets enhanced for the excited initial state, is within the 1 sigma bound of Planck 2015 results for a large field model, f(NL) similar or equal to 4.17, but is reachable by future CMB experiments. The amplitude of the local non-Gaussianity modulates around this mean value, depending on the angle that the correlated patches on the 2d CMB surface make with the preferred direction. The amount of variation is minimized for the configuration in which the short and long wavelength modes are around the preferred pole and vertical bar(k(3)) over right arrow vertical bar approximate to vertical bar(k) over right arrow (l approximate to 10)vertical bar << vertical bar(k(1)) over right arrow vertical bar approximate to vertical bar(k(2)) over right arrow vertical bar approximate to vertical bar(k) over right arrow (l approximate to 2500)vertical bar with f(NL)(min) approximate to 3.64. The maximum occurs when these modes are at the antipode of the preferred pole, f(NL)(max) approximate to 4.81. The difference of non-Gaussianity between these two configurations is as large as similar or equal to 1.17, which can be used to distinguish this scenario from other scenarios that try to explain the observed hemispherical asymmetry.

  • 5. Beltran Jimenez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Helsinki Institute of Physics, Finland; Helsinki University, Finland.
    Teleparallel Palatini theories2018In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 8, article id 039Article in journal (Refereed)
    Abstract [en]

    The Palatini formalism, which assumes the metric and the affine connection as independent variables, is developed for gravitational theories in flat geometries. We focus on two particularly interesting scenarios. First, we fix the connection to be metric compatible, as done in the usual teleparallel theories, but we follow a completely covariant approach by imposing the constraints with suitable Lagrange multipliers. For a general quadratic theory we show how torsion naturally propagates and we reproduce the Teleparallel Equivalent of General Relativity as a particular quadratic action that features an additional Lorentz symmetry. We then study the much less explored theories formulated in a geometry with neither curvature nor torsion, so that all the geometrical information is encoded in the non-metricity. We discuss how this geometrical framework leads to a purely inertial connection that can thus be completely removed by a coordinate gauge choice, the coincident gauge. From the quadratic theory we recover a simpler formulation of General Relativity in the form of the Einstein action, which enjoys an enhanced symmetry that reduces to a second linearised diffeomorphism at linear order. More general theories in both geometries can be formulated consistently by taking into account the inertial connection and the associated additional degrees of freedom. As immediate applications, the new cosmological equations and their Newtonian limit are considered, where the role of the lapse in the consistency of the equations is clarified, and the Schwarzschild black hole entropy is computed by evaluating the corresponding Euclidean action. We discuss how the boundary terms in the usual formulation of General Relativity are related to different choices of coordinates in its coincident version and show that in isotropic coordinates the Euclidean action is finite without the need to introduce boundary or normalisation terms. Finally, we discuss the double-copy structure of the gravity amplitudes and the bootstrapping of gravity within the framework of coincident General Relativity.

  • 6. Beltrán Jiménez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Coincident general relativity2018In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, no 4, article id 044048Article in journal (Refereed)
    Abstract [en]

    The metric-affine variational principle is applied to generate teleparallel and symmetric teleparallel theories of gravity. From the latter we discover an exceptional class which is consistent with a vanishing affine connection. Based on this remarkable property, this work proposes a simpler geometrical formulation of general relativity that is oblivious to the affine spacetime structure, thus fundamentally depriving gravity of any inertial character. The resulting theory is described by the Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 field theory, where an extra symmetry is now manifest, possibly related to the double-copy structure of the gravity amplitudes. This construction also provides a novel starting point for modified gravity theories, and the paper presents new and simple generalizations where analytical self-accelerating cosmological solutions arise naturally in the early-and late-time Universe.

  • 7. Beltrán Jiménez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Estonia; National Institute of Chemical Physics and Biophysic, Estonia.
    Pekar, Simon
    Cosmology in f (Q) geometry2020In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 101, no 10, article id 103507Article in journal (Refereed)
    Abstract [en]

    The universal character of the gravitational interaction provided by the equivalence principle motivates a geometrical description of gravity. The standard formulation of general relativity a la Einstein attributes gravity to the spacetime curvature, to which we have grown accustomed. However, this perception has masked the fact that two alternative, though equivalent, formulations of general relativity in flat spacetimes exist, where gravity can be fully ascribed either to torsion or to nonmetricity. The latter allows a simpler geometrical formulation of general relativity that is oblivious to the affine spacetime structure. Generalizations along this line permit us to generate teleparallel and symmetric teleparallel theories of gravity with exceptional properties. In this work we explore modified gravity theories based on nonlinear extensions of the nonmetricity scalar. After presenting some general properties and briefly studying some interesting background cosmologies (including accelerating solutions with relevance for inflation and dark energy), we analyze the behavior of the cosmological perturbations. Tensor perturbations feature a rescaling of the corresponding Newton's constant, while vector perturbations do not contribute in the absence of vector sources. In the scalar sector we find two additional propagating modes, hinting that f(Q) theories introduce, at least, 2 additional degrees of freedom. These scalar modes disappear around maximally symmetric backgrounds because of the appearance of an accidental residual gauge symmetry corresponding to a restricted diffeomorphism. We finally discuss the potential strong coupling problems of these maximally symmetric backgrounds caused by the discontinuity in the number of propagating modes.

  • 8. Beltrán Jiménez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Estonia.
    The canonical frame of purified gravity2019In: International Journal of Modern Physics D, ISSN 0218-2718, Vol. 28, no 14, article id 1944012Article in journal (Refereed)
    Abstract [en]

    In the recently introduced gauge theory of translations, dubbed Coincident General Relativity (CGR), gravity is described with neither torsion nor curvature in the spacetime affine geometry. The action of the theory enjoys an enhanced symmetry and avoids the second derivatives that appear in the conventional Einstein-Hilbert action. While it implies the equivalent classical dynamics, the improved action principle can make a difference in considerations of energetics, thermodynamics and quantum theory. This paper reports on possible progress in those three aspects of gravity theory. In the so-called purified gravity, (1) energy-momentum is described locally by a conserved, symmetric tensor, (2) the Euclidean path integral is convergent without the addition of boundary or regulating terms and (3) it is possible to identify a canonical frame for quantization.

  • 9. Beltrán Jiménez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Estonia.
    The Geometrical Trinity of Gravity2019In: Universe, E-ISSN 2218-1997, Vol. 5, no 7, article id 173Article in journal (Refereed)
    Abstract [en]

    The geometrical nature of gravity emerges from the universality dictated by the equivalence principle. In the usual formulation of General Relativity, the geometrisation of the gravitational interaction is performed in terms of the spacetime curvature, which is now the standard interpretation of gravity. However, this is not the only possibility. In these notes, we discuss two alternative, though equivalent, formulations of General Relativity in flat spacetimes, in which gravity is fully ascribed either to torsion or to non-metricity, thus putting forward the existence of three seemingly unrelated representations of the same underlying theory. Based on these three alternative formulations of General Relativity, we then discuss some extensions.

  • 10. Beltrán Jiménez, Jose
    et al.
    Heisenberg, Lavinia
    Koivisto, Tosmi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Cosmology for quadratic gravity in generalized Weyl geometry2016In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 4, article id 046Article in journal (Refereed)
    Abstract [en]

    A class of vector-tensor theories arises naturally in the framework of quadratic gravity in spacetimes with linear vector distortion. Requiring the absence of ghosts for the vector field imposes an interesting condition on the allowed connections with vector distortion: the resulting one-parameter family of connections generalises the usual Weyl geometry with polar torsion. The cosmology of this class of theories is studied, focusing on isotropic solutions wherein the vector field is dominated by the temporal component. De Sitter attractors are found and inhomogeneous perturbations around such backgrounds are analysed. In particular, further constraints on the models are imposed by excluding pathologies in the scalar, vector and tensor fluctuations. Various exact background solutions are presented, describing a constant and an evolving dark energy, a bounce and a self-tuning de Sitter phase. However, the latter two scenarios are not viable under a closer scrutiny.

  • 11. Beltrán Jiménez, Jose
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Modified Gravity with Vector Distortion and Cosmological Applications2017In: Universe, E-ISSN 2218-1997, Vol. 3, no 2, article id 47Article in journal (Refereed)
    Abstract [en]

    We briefly review the basics of Weyl geometry and its natural extension by a general linear distortion of the metric connection by a vector field. A special class of the connections has torsion but retains the Weyl's semi-metricity condition. We present ghost-free gravitational theories in this geometrical setup and highlight their possible cosmological applications, such as new self-tuning solutions and new bouncing solutions found in the quadratic-curvature theories. The vector distortion can mimic the cosmological effects of dark matter.

  • 12. Biswas, Tirthabir
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mazumdar, Anupam
    Atick-Witten Hagedorn conjecture, near scale-invariant matter and blue-tilted gravity power spectrum2014In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 8, p. 116-Article in journal (Refereed)
    Abstract [en]

    We will provide an interesting new mechanism to generate almost scale invariant seed density perturbations with a red spectrum, while keeping the gravitational wave spectrum blue-tilted in a stringy thermal contracting phase at temperatures beyond the Hagedorn temperature. This phase is often referred to as the Hagedorn phase where the free energy has been conjectured by Atick and Witten to grow more slowly than ordinary radiation. The primordial fluctuations are created by the statistical thermal fluctuations determined by the partition function, rather than quantum vacuum driven fluid dynamical fluctuations. Our mechanism assumes a non-singular bounce.

  • 13. Capozziello, Salvatore
    et al.
    Harko, Tiberiu
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Lobo, Francisco S. N.
    Olmo, Gonzalo J.
    Hybrid Metric-Palatini Gravity2015In: Universe, E-ISSN 2218-1997, Vol. 1, no 2, p. 199-238Article, review/survey (Refereed)
    Abstract [en]

    Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter.

  • 14. Carloni, Sante
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Lobo, Francisco S. N.
    Dynamical system analysis of hybrid metric-Palatini cosmologies2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, no 6, article id 064035Article in journal (Refereed)
    Abstract [en]

    The so-called f(X) hybrid metric-Palatini gravity, where X = R + T, with T the stress-energy trace and R the Ricci scalar, presents a unique viable generalization of the f(R) theories within the metric-affine formalism. In this paper, the cosmology of the f(X) theories is studied using the dynamical system approach. The method consists of formulating the propagation equation in terms of suitable (expansion-normalized) variables as an autonomous system. The fixed points of the system then represent exact cosmological solutions described by power law or de Sitter expansion. The formalism is applied to two classes of f(X) models, revealing both standard cosmological fixed points and new accelerating solutions that can be attractors in the phase space. In addition, the fixed point with vanishing expansion rate is considered with special care in order to characterize the stability of Einstein static spaces and bouncing solutions.

  • 15. Conroy, Aindriu
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mazumdar, Anupam
    Teimouri, Ali
    Generalized quadratic curvature, non-local infrared modifications of gravity and Newtonian potentials2015In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 32, no 1, article id 015024Article in journal (Refereed)
    Abstract [en]

    Metric theories of gravity are studied, beginning with a general action that is quadratic in curvature and allows arbitrary inverse powers of the d' Alembertian operator, resulting in infrared non-local extensions of general relativity. The field equations are derived in full generality and their consistency is checked by verifying the Bianchi identities. The weak-field limit is computed and a straightforward algorithm is presented to infer the post-Newtonian corrections directly from the action. This is then applied to various infrared gravity models including non-local Rf (R/square) dark energy and non-local massive gravity models. Generically, the Newtonian potentials are not identical and deviate from the 1/r behaviour at large distances. However, the former does not occur in a specific class of theories that does not introduce additional degrees of freedom in flat spacetime. A new non-local model within this class is proposed, defined by the exponential of the inverse d' Alembertian. This model exhibits novel features, such as the weakening of the gravity in the infrared, suggesting de-gravitation of the cosmological constant.

  • 16. Dialektopoulos, Konstantinos F.
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Estonia; National Institute of Chemical Physics and Biophysics, Estonia.
    Capozziello, Salvatore
    Noether symmetries in symmetric teleparallel cosmology2019In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 7, article id 606Article in journal (Refereed)
    Abstract [en]

    We consider a general theory of all possible quadratic, first-order derivative terms of the non-metricity tensor in the framework of Symmetric Teleparallel Geometry. We apply the Noether Symmetry Approach to classify those models that are invariant under point transformations in a cosmological background and we use the symmetries of these models to reduce the dynamics of the system in order to find analytical solutions.

  • 17. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). National Institute of Chemical Physics and Biophysics, Estonia.
    Cosmological perturbations in modified teleparallel gravity models2018In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 11, article id 012Article in journal (Refereed)
    Abstract [en]

    Cosmological perturbations are considered in f (T) and in scalar-torsion f (phi) T teleparallel models of gravity. Full sets of linear perturbation equations are accurately derived and analysed at the relevant limits. Interesting features of generalisations to other teleparallel models, spatially curved backgrounds, and rotated tetrads are pointed out.

  • 18. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oslo, Norway.
    Effectively nonlocal metric-affine gravity2016In: Physical Review D, ISSN 2470-0010, Vol. 93, no 6, article id 064081Article in journal (Refereed)
    Abstract [en]

    In metric-affine theories of gravity such as the C-theories, the spacetime connection is associated to a metric that is nontrivially related to the physical metric. In this article, such theories are rewritten in terms of a single metric, and it is shown that they can be recast as effectively nonlocal gravity. With some assumptions, known ghost-free theories with nonsingular and cosmologically interesting properties may be recovered. Relations between different formulations are analyzed at both perturbative and nonperturbative levels, taking carefully into account subtleties with boundary conditions in the presence of integral operators in the action, and equivalences between theories related by nonlocal redefinitions of the fields are verified at the level of equations of motion. This suggests a possible geometrical interpretation of nonlocal gravity as an emergent property of non-Riemannian spacetime structure.

  • 19. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    On the covariance of teleparallel gravity theories2017In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 34, no 14, article id 145013Article in journal (Refereed)
    Abstract [en]

    The basics of teleparallel gravity and its extensions are reviewed with particular emphasis on the problem of the Lorentz-breaking choice of connection in pure-tetrad versions of the theories. Various possible ways to covariantise such models are discussed. A by-product is a new form of f (T) field equations.

  • 20. Harko, Tiberiu
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Lobo, Francisco S. N.
    Olmo, Gonzalo J.
    Rubiera-Garcia, Diego
    Coupling matter in modified Q gravity2018In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, no 8, article id 084043Article in journal (Refereed)
    Abstract [en]

    We present a novel theory of gravity by considering an extension of symmetric teleparallel gravity. This is done by introducing, in the framework of the metric-affine formalism, a new class of theories where the nonmetricity Q is nonminimally coupled to the matter Lagrangian. More specifically, we consider a Lagrangian of the form L similar to f(1)(Q) + f(2)(Q)L-M, where f(1) and f(2) are generic functions of Q, and L-M is the matter Lagrangian. This nonminimal coupling entails the nonconservation of the energy-momentum tensor, and consequently the appearance of an extra force. The formulation of the gravity sector in terms of the Q instead of the curvature may result in subtle improvements of the theory. In the context of nonminimal matter couplings, we are therefore motivated to explore whether the new geometrical formulation in terms of the Q, when implemented also in the matter sector, would allow more universally consistent and viable realizations of the nonminimal coupling. Furthermore, we consider several cosmological applications by presenting the evolution equations and imposing specific functional forms of the functions f(1)(Q) and f(2)(Q), such as power-law and exponential dependencies of the nonminimal couplings. Cosmological solutions are considered in two general classes of models, and found to feature accelerating expansion at late times.

  • 21. Iosifidis, Damianos
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Finland; National Institute of Chemical Physics and Biophysics, Estonia; Helsinki University, Finland.
    Scale Transformations in Metric-Affine Geometry2019In: Universe, E-ISSN 2218-1997, Vol. 5, no 3, article id 82Article in journal (Refereed)
    Abstract [en]

    This article presents an exhaustive classification of metric-affine theories according to their scale symmetries. First it is clarified that there are three relevant definitions of a scale transformation. These correspond to a projective transformation of the connection, a rescaling of the orthonormal frame, and a combination of the two. The most general second order quadratic metric-affine action, including the parity-violating terms, is constructed in each of the three cases. The results can be straightforwardly generalised by including higher derivatives, and implemented in the general metric-affine, teleparallel, and symmetric teleparallel geometries.

  • 22. Jimenez, Jose Beltran
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oslo, Norway.
    Extended Gauss-Bonnet gravities in Weyl geometry2014In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 31, no 13, p. 135002-Article in journal (Refereed)
    Abstract [en]

    In this paper we consider an extended Gauss-Bonnet gravity theory in arbitrary dimensions and in a space provided with a Weyl connection, which is torsion-free but non-metric-compatible, the non-metricity tensor being determined by a vector field. The action considered consists of the usual Einstein-Hilbert action plus all the terms quadratic in the curvature that reduce to the usual Gauss-Bonnet term for vanishing Weyl connection, i.e., when only the Levi-Civita part of the connection is present. We expand the action in terms of Riemannian quantities and obtain vector-tensor theories. We find that all the free parameters only appear in the kinetic term of the vector field, so two branches are possible: one with a propagating vector field and another one where the vector field does not propagate. We focus on the propagating case. We find that in four dimensions, the theory is equivalent to Einstein's gravity plus a Proca field. This field is naturally decoupled from matter, so it represents a natural dark matter candidate. Also for d = 4, we discuss a non-trivial cubic term in the curvature that can be constructed without spoiling the second-order nature of the field equations, because it leads to the vector-tensor Horndeski interaction. In arbitrary dimensions, the theory becomes more involved. We show that, even though the vector field presents kinetic interactions which do not have U(1) symmetry, there are no additional propagating degrees of freedom with respect to the usual massive case. We show that, interestingly, this relies on the fact that the corresponding Stuckelberg field belongs to a specific class within the general Horndeski theories. Finally, since Weyl geometries provide the natural ground on which to build scale invariant theories, we apply the usual Weyl gauging in order to make the Horndeski action locally scale invariant, and discuss new terms that can be added.

  • 23. Jiménez, Jose Beltrán
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Spacetimes with vector distortion: Inflation from generalised Weyl geometry2016In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 756, p. 400-404Article in journal (Refereed)
    Abstract [en]

    Spacetime with general linear vector distortion is introduced. Thus, the torsion and the nonmetricity of the affine connection are assumed to be proportional to a vector field (and not its derivatives). The resulting two-parameter family of non-Riemannian geometries generalises the conformal Weyl geometry and some other interesting special cases. Taking into account the leading nonlinear correction to the Einstein-Hilbert action results uniquely in the one-parameter extension of the Starobinsky inflation known as the alpha-attractor. The most general quadratic curvature action introduces, in addition to the canonical vector kinetic term, novel ghost-free vector-tensor interactions.

  • 24.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    An integrable geometrical foundation of gravity2018In: International Journal of Geometric Methods in Modern Physics (IJGMMP), ISSN 0219-8878, Vol. 15, article id 1840006Article in journal (Refereed)
    Abstract [en]

    In a talk at the conference Geometrical Foundations of Gravity at Tartu, 2017, it was suggested that the affine spacetime connection could be associated with purely fictitious forces. This leads to gravitation in a flat and smooth geometry. Fermions are found to nevertheless couple with the metrical connection and a phase gauge field. The theory is reviewed in this proceeding in a Palatini, and in a metric-affine gauge formulation.

  • 25.
    Koivisto, Tomi
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Estonia; National Institute of Chemical Physics and Biophysics, Estonia.
    Hohmann, Manuel
    Złośnik, Tom
    The General Linear Cartan Khronon2019In: Universe, E-ISSN 2218-1997, Vol. 5, no 7, article id 168Article in journal (Refereed)
    Abstract [en]

    A Cartan geometry of the General Linear symmetry is formulated by dividing out the displacements from the group. The resulting action is quadratic in curvature, polynomial in all the (minimal) variables, and describes an observer space that-in the symmetry-broken phase-reproduces the predictions of General Relativity in the presence of dark matter.

  • 26.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Helsinki Institute of Physics, Finland; Helsinki University, Finland.
    Nyrhinen, Hannu J.
    Stability of disformally coupled accretion disks2017In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 92, no 10, article id 105301Article in journal (Refereed)
    Abstract [en]

    The no-hair theorem postulates that the only externally observable properties of a black hole are its mass, its electric charge, and its angular momentum. In scalar-tensor theories of gravity, a matter distribution around a black hole can lead to the so called 'spontaneous scalarisation' instability that triggers the development of scalar hair. In the Brans-Dicke type theories, this effect can be understood as a result of tachyonic effective mass of the scalar field. Here we consider the instability in the generalised class of scalar-theories that feature non-conformal, i.e. 'disformal', couplings to matter. Such theories have gained considerable interest in the recent years and have been studied in a wide variety of systems, both cosmological and astrophysical. In view of the prospects of gravitational wave astronomy, it is relevant to explore the implications of the theories in the strong-gravity regime. In this article, we concentrate on the spontaneous scalarisation of matter configurations around Schwarzschild and Kerr black holes. We find that in the more generic scalar-tensor theories, the instability of the Brans-Dicke theory can be enhanced, suggesting violations of the no-hair theorem. On the other hand, we find that, especially if the coupling is very strong, or if the gradients in the matter distribution are negligible, the disformal coupling tends to stabilise the system.

  • 27.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Saridakis, Emmanuel N.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Pontificia Universidad Catolica de Valparaiso, Chile.
    Tamanini, Nicola
    Scalar-fluid theories: cosmological perturbations and large-scale structure2015In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 9, article id 047Article in journal (Refereed)
    Abstract [en]

    Recently a new Lagrangian framework was introduced to describe interactions between scalar fields and relativistic perfect fluids. This allows two consistent generalizations of coupled quintessence models: non-vanishing pressures and a new type of derivative interaction. The implications of these to the formation of cosmological large-scale structure are uncovered here at the linear order. The full perturbation equations in the two cases are derived in a unified formalism and their Newtonian, quasi-static limit is studied analytically. Requiring the absence of an effective sound speed term in the coupled dark matter fluid restricts the Lagrangian to be a linear function of the matter number density. This leaves new potentially viable classes of both algebraically and derivatively interacting models wherein the coupling may impact the background expansion dynamics and imprint new signatures into the large-scale structure.

  • 28.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Tamanini, Nicola
    A note on viability of nonminimally coupled f(R) theory2016In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 48, no 7, article id 97Article in journal (Refereed)
    Abstract [en]

    Consistency conditions for nonminimally coupled f(R) theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds.

  • 29.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Urban, Federico R.
    Disformal vectors and anisotropies on a warped brane Hulluilla on Halvat Huvit2015In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 3, article id 003Article in journal (Refereed)
    Abstract [en]

    The Maxwell action is conformally invariant and classically ignorant of conformally flat metrics. However, if the vector lives in a disformal metric - as it does if residing upon a moving brane - this is no longer true. The disformal coupling is then mediated by a Dirac-Born-Infeld scalar field. Here a systematic dynamical system analysis is developed for anisotropic Bianchi I cosmology with a massive disformally coupled vector field. Several new fixed points are found, including anisotropic scaling solutions. The formalism here presented can be conveniently applied to general scenarios with or without extra dimensional motivations. This is illustrated here by performing a complete analysis with the assumption that both the potentials and the warp factor for the brane are (nearly) exponential. In that case, the anisotropic fixed points are either not attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, viable classes of models exist where isotropy is retained due to rapid oscillations of the vector field, thus providing a possible realisation of disformally interacting massive dark matter.

  • 30.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Urban, Federico R.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Université Libre de Bruxelles, Belgium.
    Doubly-boosted vector cosmologies from disformal metrics2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 90, no 9, article id 095301Article in journal (Refereed)
    Abstract [en]

    A systematic dynamical system approach is applied to study the cosmology of anisotropic Bianchi I universes in which a vector field is assumed to operate on a disformal frame. This study yields a number of new fixed points, among which are anisotropic scaling solutions. Within the simplifying assumption of (nearly) constant-slope potentials these are either not stable attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, some solutions do have an appeal for cosmological applications in that isotropy is retained due to rapid oscillations of the vector field. Such cosmological scenarios could describe physics beyond standard model such as extra dimensional models that predict disformal couplings between vector and scalar fields.

  • 31.
    Koivisto, Tomi
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Tartu, Estonia; National Institute of Chemical Physics and Biophysics, Estonia; Helsinki Institute of Physics, Finland; Helsinki University, Finland.
    Tsimperis, Georgios
    Stockholm University, Faculty of Science, Department of Physics.
    The Spectrum of Teleparallel Gravity2019In: Universe, E-ISSN 2218-1997, Vol. 5, no 3, article id 80Article in journal (Refereed)
    Abstract [en]

    The observer's frame is the more elementary description of the gravitational field than the metric. The most general covariant, even-parity quadratic form for the frame field in arbitrary dimension generalises the New General Relativity by nine functions of the d'Alembertian operator. The degrees of freedom are clarified by a covariant derivation of the propagator. The consistent and viable models can incorporate an ultra-violet completion of the gravity theory, an additional polarisation of the gravitational wave, and the dynamics of a magnetic scalar potential.

  • 32. Nersisyan, Henrik
    et al.
    Akrami, Yashar
    Amendola, Luca
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Rubio, Javier
    Dynamical analysis of R1/square R-2 cosmology: Impact of initial conditions and constraints from supernovae2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 4, article id 043531Article in journal (Refereed)
    Abstract [en]

    We discuss the cosmological implications of the R square R-2 nonlocal modification to standard gravity. We relax the assumption of special initial conditions in the local formulation of the theory, perform a full phase-space analysis of the system, and show that the late-time cosmology of the model exhibits two distinct evolution paths, on which a large range of values for the present equation of state can be reached. We then compare the general solutions to supernovae data and place constraints on the parameters of the model. In particular, we find that the mass parameter of the theory should be smaller than 1.2 in Hubble units.

  • 33. Solomon, Adam R.
    et al.
    Akrami, Yashar
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Linear growth of structure in massive bigravity2014In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 10, p. 066-Article in journal (Refereed)
    Abstract [en]

    The ghost-free theory of massive gravity with two dynamical metrics has been shown to produce viable cosmological expansion, where the late-time acceleration of the Universe is due to the finite range of the gravitational interaction rather than a nonzero cosmological constant. Here the cosmological perturbations are studied in this theory. The full perturbation equations are presented in a general gauge and analyzed, focusing on subhorizon scales in the quasistatic limit during the matter-dominated era. An evolution equation for the matter inhomogeneities and the parameters quantifying the deviations from general relativistic structure formation are expressed in terms of five functions whose forms are determined directly by the coupling parameters in the theory. The evolution equation has a similar structure to Horndeski-type scalar-tensor theories, exhibiting a modified growth rate and scale-dependence at intermediate wavenumbers. Predictions of the theory are confronted with observational data on both background expansion and large-scale structure, although care must be taken to ensure a model is stable. It is found that while the stable models tit the data well, they feature deviations from the standard cosmology that could be detected or ruled out by near-future experiments.

  • 34. Solomon, Adam R.
    et al.
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Akrami, Yashar
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Könnig, Frank
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cosmological viability of massive gravity with generalized matter coupling2015In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 4, article id 027Article in journal (Refereed)
    Abstract [en]

    There is a no-go theorem forbidding flat and closed FLRW solutions in massive gravity on a flat reference metric, while open solutions are unstable. Recently it was shown that this no-go theorem can be overcome if at least some matter couples to a hybrid metric composed of both the dynamical and the fixed reference metric. We show that this is not compatible with the standard description of cosmological sources in terms of effective perfect fluids, and the predictions of the theory become sensitive either to the detailed field-theoretical modelling of the matter content or to the presence of additional dark degrees of freedom. This is a serious practical complication. Furthermore, we demonstrate that viable cosmological background evolution with a perfect fluid appears to require the presence of fields with highly contrived properties. This could be improved if the equivalence principle is broken by coupling only some of the fields to the composite metric, but viable self-accelerating solutions due only to the massive graviton are difficult to obtain. These problems can be avoided by giving the reference metric dynamics.

  • 35.
    Sundell, Peter
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Anisotropic cosmology and inflation from a tilted Bianchi IX model2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, no 12, article id 123529Article in journal (Refereed)
    Abstract [en]

    The dynamics of the tilted axisymmetric Bianchi IX cosmological models are explored allowing energy flux in the source fluid. The Einstein equations and the continuity equation are presented treating the equation of state w and the tilt angle of the fluid lambda as time-dependent functions, but when analyzing the phase space w and lambda are considered free parameters and the shear, the vorticity and the curvature of the spacetime span a three-dimensional phase space that contains seven fixed points. One of them is an attractor that inflates the universe anisotropically, thus providing a counterexample to the cosmic no-hair conjecture. Also, examples of realistic though fine-tuned cosmologies are presented wherein the rotation can become significant towards the present epoch but the shear stays within the observational bounds. The examples suggest that the model used here can explain the parity-violating anomalies of the cosmic microwave background. The result significantly differs from an earlier study, where a nonaxisymmetric Bianchi IX type model with a tilted perfect dust source was found to induce too much shear for observationally significant vorticity.

  • 36. van de Bruck, Carsten
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Longden, Chris
    Disformally coupled inflation2016In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 3, article id 006Article in journal (Refereed)
    Abstract [en]

    A disformal coupling between two scalar fields is considered in the context of cosmological inflation. The coupling introduces novel derivative interactions mixing the kinetic terms of the fields but without introducing superluminal or unstable propagation of the two scalar fluctuation modes. Though the typical effect of the disformal coupling is to inhibit one of the fields from inflating the universe, the energy density of the other field can drive viable near Sitter-inflation in the presence of nontrivial disformal dynamics, in particular when one assumes exponential instead of power-law form for the couplings. The linear perturbation equations are written for the two-field system, its canonical degrees of freedom are quantised, their spectra are derived and the inflationary predictions are reported for numerically solved exponential models. A generic prediction is low tensor-to-scalar ratio.

  • 37. van de Bruck, Carsten
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Longden, Chris
    Non-Gaussianity in multi-sound-speed disformally coupled inflation2017In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 2, p. 1-29, article id 029Article in journal (Refereed)
    Abstract [en]

    Most, if not all, scalar-tensor theories are equivalent to General Relativity with a disformally coupled matter sector. In extra-dimensional theories such a coupling can be understood as a result of induction of the metric on a brane that matter is confined to. This article presents a first look at the non-Gaussianities in disformally coupled inflation, a simple two-field model that features a novel kinetic interaction. Cases with both canonical and Dirac-Born-Infeld (DBI) kinetic terms are taken into account, the latter motivated by the possible extra-dimensional origin of the disformality. The computations are carried out for the equilateral con figuration in the slow-roll regime, wherein it is found that the non-Gaussianity is typically rather small and negative. This is despite the fact that the new kinetic interaction causes the perturbation modes to propagate with different sounds speeds, which may both significantly deviate from unity during inflation.

  • 38. Złośnik, Tom
    et al.
    Urban, Federico
    Marzola, Luca
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). National Institute of Chemical Physics and Biophysics, Estonia.
    Spacetime and dark matter from spontaneous breaking of Lorentz symmetry2018In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 35, no 23, article id 235003Article in journal (Refereed)
    Abstract [en]

    It is shown that a spontaneously-broken gauge theory of the Lorentz group contains Ashtekar's chiral formulation of general relativity accompanied by dust. From this perspective, gravity is described entirely by a connection omega valued in the Lie algebra of the complexified Lorentz group and a Lorentz-charged scalar field tau. The model is 'pre-geometric' in the sense that the spacetime metric may be constructed only in the symmetry-broken regime. We speculate on the extent to which this dust may account for dark matter and on the behaviour of the theory in regimes where the symmetry is not broken.

1 - 38 of 38
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